1. Field of the Invention
The present invention relates to rising control for an image heating apparatus.
2. Description of the Related Art
An image forming apparatus using an electrophotographic process includes a fixing device. The fixing device fuses and fixes toner to a recording material by applying heat and pressure while nipping and transporting the recording material, on which the toner containing a resin, a magnetic substance, a coloring matter, etc. is electrostatically borne, in a press-contacting portion (nip portion) between heating means (a roller, an endless belt member, etc.) and pressurizing means (a roller, an endless belt member, etc.) which are maintained in mutual pressure-contact and in rotation.
FIG. 1 is a schematic view showing a structure of the image forming apparatus.
The image forming apparatus includes a photosensitive drum 1 that rotates in a direction shown by an arrow V1. After a photoconductive layer on a surface of the photosensitive drum 1 is uniformly charged by a primary charger 2, a light image exposure 3 of an original (not shown) causes an electrostatic latent image thereof to be formed on the surface of the photosensitive drum 1. According to the rotation of the photosensitive drum 1, the electrostatic latent image reaches a location corresponding to a developing device 4 and is developed by toner supplied from the developing device 4, thereby being visualized as a toner image.
Then, according to the rotation of the photosensitive drum 1, the obtained toner image advances to a transferring part arranged with a transferring charger 6.
Meanwhile, a recording material P that travels in synchronization with the photosensitive drum 1 is guided along a transport path 5 to reach the transferring part. In the transferring part, the recording material P adheres to the surface of the photosensitive drum 1.
In the transferring part, charges having a polarity reverse to that of the toner are imparted to the recording material P by the transferring charger 6, and then, the toner image on the surface of the photosensitive drum 1 is transferred onto the recording material P due to an electrostatic force.
On a downstream side of the transferring charger 6 along the travelling direction of the recording material, a separation charger 7 is arranged. From the recording material P onto which an unfixed toner image has been transferred, the charges are eliminated due to corona discharge by the separation charger 7. Thus, the recording material P has no longer an adsorbing force, and is then separated from the photosensitive drum 1 with the assistance of the elasticity inherent in the recording material P per se and also the self weight of the recording material P.
The separated recording material P is transported by a transport part 8 composed of a conveyor belt to the fixing device composed of a pair of a fixing roller 10 and a pressure roller 120 as the subsequent rotary member pair. While passing through the nip between the fixing roller 10 and the pressure roller 120, the unfixed toner image is fixed to the recording material P.
In that case, a rotating speed of the peripheral surface of the fixing roller 10 which is transporting the recording material P and a rotating speed of the peripheral surface of the photosensitive drum 1 are generally set to be different from each other. In general, the fixing roller 10 is set to be rotated slightly slower than the photosensitive drum 1. This is because the above setting can suppress a shock that is caused when a leading end of the recording material P enters the nip between the fixing roller 10 and the pressure roller 120, to thereby prevent deviation from occurring in the toner image transferred onto a trailing end of the recording material P still passing through the transferring part.
The fixing device will be described in further detail by reference to FIG. 2.
The fixing roller 10 includes an aluminum core 103 coated with a PFA tube 101 as a release layer.
Inside the aluminum core 103, a main heater 109 and a sub heater 108 are arranged.
The main heater 109 and the sub heater 108 are turned ON/OFF to thereby heat the fixing roller 10. In other words, the fixing device is structured so as to maintain only the fixing roller 10 at a high temperature. Note that the fixing device may be structured so as to maintain not only fixing roller 10 but also the pressure roller 120 at a high temperature by providing a heater inside the pressure roller 120 as well. Alternatively, the fixing device may be structured so as to maintain only the pressure roller 120 at a high temperature.
Outside the PFA tube 101 of the fixing roller 10, a thermistor 102 is arranged. Thus, an amount of energization to the main heater 109 and the sub heater 108 is controlled by a control circuit so as to maintain a constant temperature.
In a standby state, the temperature of the fixing roller 10 is controlled to a temperature that allows fixing.
In order to form the nip in which the recording material is nipped and transported, the pressure roller 120 is pressurized against the fixing roller 10.
The pressure roller 120 includes an iron core 106, silicone rubber 105 arranged on the iron core 106, and a PFA tube 104 as a release layer arranged as a surface layer.
The fixing device described above has a low power mode in order to reduce power consumption. In the low power mode, the temperature of the fixing roller 10 is controlled to a temperature lower than a standby temperature maintained in the standby state that allows image forming in which an image forming start signal is ready to be received. In order to recover the standby state from the low power mode, the temperature of the fixing roller 10 is raised to the standby temperature to effect an image formable state in which image forming is ready.
It is desired to provide an image forming apparatus in which energy consumption can be reduced in total by increasing the frequency of entering the above-mentioned low power mode while the usability is improved by minimizing a time for recovery of the image formable state from the low power mode. Thus, it is preferable to recover the image formable state from the low power mode within a given time.
Therefore, for the purpose of recovery within a given time, the structure is proposed in JP 2000-200013 A, which functions to measure the time needed to reach the standby temperature during the rising time from the power ON until the standby state, and according to the time thus measured, change the control temperature in the low power mode.
However, the environment at the above-mentioned rising time and the environment at the time of recovery form the low power mode are different in many cases. For example, the temperature of the fixing roller at the time of power ON may be lower than a controlled temperature thereof at the time of the low power mode. Accordingly, with the structure disclosed in JP 2000-200013 A, the recovery is impossible in a given time due to variation of environments.
On the other hand, in the low power mode, a relatively high temperature is set in many cases. Therefore, compared with the case of rising from the room temperature, it can be taken into consideration that heat radiation from the fixing roller is reduced in amount because the entire fixing device has been heated to a sufficient level, and the heater is continuously in an ON state during the recovery. Thus, even before reaching the standby temperature, the fixing roller is already in a state that allows fixing in many cases. Accordingly, in the case of rising from the low power mode, there is no need to effect the image formable state under the same conditions as those of the case of rising from the room temperature. As described above, there is no need to wait until the standby temperature is reached while the state that allows fixing is effected as in the conventional art, so that the time until the image formable state is recovered from the low power mode can be reduced.